As shown in FIG. 1, an optical network 8 is comprised of a plurality of nodes 10a and 10b linked together via an optical fiber 12.
The construction and operation of nodes is well known in the art.
Wave-division multiplexing is a type of multiplexing in which two or more optical carriers are multiplexed onto a single optical fiber by using different wavelengths (that is, colors) of laser light.
As shown in FIG. 1, the Nodes 10a and 10b are provided with multiple transceivers 14a, 14b and 14c; a multiplexer/demultiplexer 16; and an optical amplifier 18. The transceiver 14a is coupled to the optical amplifier 18. The optical amplifier includes an Erbium Doped Fiber Amplifier, and/or an optical add/drop multiplexer, such as a fixed optical add/drop multiplexer, or a reconfigurable optical add/drop multiplexer. The transceivers 14b and 14c are coupled to the optical amplifier 18 via the multiplexer/demultiplexer 16. Each of the transceivers 14a, 14b and 14c transmit light in a distinct spectral band.
Amplifier and optical fiber response (transfer function) depends upon loading conditions of the optical fiber. At the end of a link, transfer functions of the line amplifier, fibers, ROADMs, etc. are accumulated. Power differences between different parts of the optical spectrum resulting from the accumulated transfer functions will be referred to as the ‘accumulated line spread’.
Shown in FIG. 2 are optical power spectra showing a first loading scenario in which the node 10a transmits a plurality of optical carriers having a first optical signal power spectrum 20, and the node 10b receives a second optical signal power spectrum 22. The first optical signal power spectrum includes a relatively flat level of amplifier noise (Amplified Spontaneous Emission (ASE), with optical data carrying signal channels 1, 2 and 3 at distinct bands within the first optical signal power spectrum 20. As can be seen in FIG. 2, the second optical signal power spectrum 22 includes increases and decreases in the level of relative power levels between the data channels 1, 2 and 3, as well as in the amplifier noise. The shape of the received optical signal power spectrum 22 is referred to as the accumulated line spread of an optical link. As will be appreciated by those skilled in the art, the increase and decrease in the level of power in the second optical signal power spectrum 22 makes it difficult to correctly read the signal at the receiving node 10b due to the changing dynamics of the fiber and amplifier response overtime and over different signal channel loading conditions.
This problem exists regardless of where in an optical signal power spectrum carrier channels are broadcast. Shown in FIG. 3 are waveforms showing a second loading scenario in which the node 10a transmits a plurality of optical carriers forming a third optical signal power spectrum 24, and the node 10b receives a fourth optical signal power spectrum 26. The third optical signal power spectrum includes a relatively constant level of power, with homogenous increases in power at channels 1, 2 and 3 at distinct bands within the third optical signal power spectrum 24. The third optical signal power spectrum 24 includes the channels 1, 2 and 3 at different optical channels than within the first optical signal power spectrum 20. As can be seen in FIG. 3, the fourth optical signal power spectrum 26 includes increases and decreases in the level of power between the optical channels 1, 2 and 3, as well as inhomogeneous power levels within the optical channels 1, 2 and 3. In addition, the accumulated line spread is different from the first loading scenario and is hard to predict.
The problem with unpredictable accumulated line spread is that performance of an optical carrier is optimized at a certain power level, balancing linear and nonlinear penalties. If the line signal loading changes (and thus the accumulated line spread), performance will degrade. For instance, the changes in effective gain, hence the received power changes of signals 1, 2, and 3 will impact the properties of these channels and the quality of the signal. Examples of such properties is the optical signal-to-noise ratio (OSNR) and the amount of degradations accumulated in the link due to the fiber nonlinearities. When the linespread changes, the power of the optical data signal may change significantly. For example, an increase in optical power may increase the nonlinearity based degradations since the nonlinear noise is proportional to the optical power. Or a decrease in optical power will degrade the resulting OSNR. Either of these events may result in degradations of the signal quality. Optical networks and link design has to accommodate design margins for such changes to absorb these kind of changes, limiting the performance and signal reach. Moreover, the dynamic range the optical networking hardware designed for has to be very large in order to accommodate low and maximum loading/capacity conditions.